1. Field of the Invention
The present invention relates to a method for perforating a sheet-shaped material or web by employing pulsed laser beams, and an apparatus for perforating such a material by the pulsed laser beams.
2. Description of the Related Art
A technique of forming plural fine holes through a sheet-shaped material (web) is widely used in a tabacco processing industry.
A cigarette equiped with a filter is produced by combining a cigarette rod with a filter rod and then wrapping a tipping paper around the cigarette rod combined with the filter rod. Normally, a pair of cigarettes with the filters are prepared by the combination of two cigarettes, and such combined cigarette is finally cut at its center to produce two pieces of cigarettes with the filter.
The tipping paper to be wrapped around the filter and also the cigarette is supplied by cutting a sheet-shaped material wound around a reel into small pieces with a predetermined size.
The tipping paper is formed with a ventilation area where plural fine holes are formed, so that tar and nicotine may be readily adsorbed by the filter in smoking, that is, a so-called dilution characteristic may be exhibited.
To achieve the above dilution characteristic, the ventilation area of the tipping paper is required to have a desired fine hole pattern composed of plural fine holes having a uniform shape and accurately positioned.
As previously described, the tipping paper is produced as a sheet-shaped material having a width corresponding to a double-length of cigarette during the cigarette producing process. Accordingly, it is preferable to form two rows of fine hole patterns which extend along a longitudinal axis of the sheet-shaped material.
Such fine hole patterns are formed by utilizing a pulsed laser beam at present. Generally, the pulsed laser beam is generated based upon the following two typical beam generating methods.
The first generating method is to pulse a continuous laser beam by means of a rotating reflecting disk.
The reflecting disk is provided at its circumference with beam reflecting elements and beam passing openings. A plurality of the reflecting disks are arranged so as to obtain a desired fine hole pattern on the sheet-shaped material. Thus, the pulsed laser beam is irradiated on the moving sheet-shaped material to thereby form arrays of fine holes along the longitudinal axis of the material.
Such a continuous laser generating method is known from, for instance, U.S. Pat. No. 4,118,619, GB patent No. 2002492 and European patent No. 21165.
The second generating method is to directly generate a pulsed laser beam.
In the second generating method, a pulsed laser beam is split by a partially light permeable mirror and a total reflecting mirror, and thereafter the split beams are irradiated onto the moving sheet-shaped material to thereby form the plural arrays of the fine hole pattern.
The above-described pulsed laser generating method is known from, for instance, GB patent No. 2124128-A.
However, in these conventional perforating techniques by employing a pulsed laser beam so as to form a desired fine hole pattern on a sheet-shaped material, a complex perforating system having many optical elements is required. Accordingly, the number of elements required for precise adjustment is necessarily increased, resulting in an expensive perforating system.
In, on the other hand, the pulsed laser generating method, a waveform of the once oscillated pulsed beam will tail for a period of about 1 msec even after an exciting current is interrupted, due to the inherent characteristic of CO.sub.2 gas laser. For this reason, the succeeding pulsed laser beam cannot be continuously oscillated at a short interval. That is, to increase a pulse oscillation frequency, a certain limitation exists to block high-speed perforating operation. Moreover, the resultant fine holes have spindle shapes. The formation of such spindle-shaped fine holes causes a problem such that adjacent spindle-shaped fine holes are coupled to newly form a large hole. As a result, it is practically impossible to provide filter tipping paper having desirable precise dilution characteristics.
The present invention has been therefore accomplished to solve the above-described conventional drawbacks, and has an object to provide a perforating method and apparatus which is simple in optical arrangement with less adjusting elements and can form desirable fine hole patterns at a high efficiency by using a large quantity of split beam obtained from a pulsed laser beam.